The present invention is particularly directed to vitrified bonded grinding wheels prepared with sol gel aluminous abrasive grain. Vitrified bonded grinding wheels are different from resinoid bonded, i.e. phenol-aldehyde, wheels in that they use a glass phase to bond the grain and thus are processed at substantially higher temperatures (about 800.degree. C. or higher for vitrified vs. about 400.degree. C. or lower for resin). Due to the higher manufacturing temperature used, vitrified bonded wheels can withstand higher temperatures in use than can resinoid bonded wheels. Also, it must be noted that sintered sol gel aluminous abrasive grain is known to be distinct from other aluminous abrasive grain, i.e. fused alumina. U.S. Pat. No. 4,543,107 discloses that vitrified wheels produced from sol gel grain must be processed at lower temperatures than similar wheels from fused alumina grain. Sol gel grain must be processed generally about 1100.degree. C. or less for conventional vitrified bonds and about 1220.degree. C. or less for more viscous high alumina and silica bonds to yield satisfactory wheels.
Silicon-containing materials have been used as abrasive grain coatings and the like to provide improved properties for resinoid bonded wheels, i.e. phenol-formaldehyde and the like, which are processed at relatively low temperature, but not with vitrified bond wheels. This is not surprising because silicon-containing materials are known to improve the water repellency of many articles and resinoid-bonded grinding wheels are known to suffer degradation due to the effects of cooling water present during use. The silicon treatments of grain for resinoid wheels effectively "waterproofs" the resinoid-bonded abrasive articles to prevent strength degradation and to increase retention of grinding grade throughout the useful life of the resinoid-bonded abrasive article. Since vitrified-bonded abrasive bodies are inherently resistant to degradation from water, such treatments have not been performed. Moreover, since vitrified-bonded bodies are subjected to extremely high temperatures for forming, one would expect any organic silane present to be destroyed by the temperatures and thus not effect the performance of the resultant grinding wheel. With conventional fused aluminous grain, the expected destruction of the silane does occur, as is shown below.
Also, vitrified bonded wheels have previously been impregnated with molten sulfur, as a grinding aid. The molten sulfur is placed on the wheel after it has been fired and thus can not affect the individual alumina grains used to form the wheel.
Cottringer et al, U.S. Pat. No. 4,623,364, and pending, commonly owned, U.S. application Ser. No. 023,346, filed Mar. 9, 1987, describe the preparation of various ceramic bodies, including abrasive grits, by heating gels of alumina monohydrate seeded with crystalline alpha alumina or other materials capable of facilitating the conversion of alumina monohydrate to crystalline alpha alumina. Such seeded sol gel alumina abrasive grits are characterized by submicron sized alpha alumina crystals, high hardness, and a greater density than the sol gel abrasive grits produced by the Leitheiser et al teachings in U.S. Pat. No. 4,518,397 which do not employ a seeding procedure. Similarly, Schwabel, U.S. Pat. No. 4,744,802, describes seeding or nucleating processes for producing dense, high hardness sol gel sintered alumina abrasive grits in which the nucleating agent is alpha ferric oxide.
For purposes of this application and the invention disclosed, the term "sol gel sintered alumina abrasive" is intended to apply to abrasives produced by the teachings of any one of U.S. Pat. Nos. 4,518,397, 4,623,364, and 4,744,802, as well as other sol gel techniques used in the art.